Compensated output solid-state differential accelerometer

ABSTRACT

A solid-state differential accelerometer employing piezotransistors for measuring acceleration or velocity of a body along a specified axis of the accelerometer. A pair of substantially identical piezotransistors are arranged back-toback along the axis through which tension and compression forces of acceleration are applied. A piezoelectrical emitter-base junction of each transistor is mechanically linked to a diaphragm portion of the transistor case that is normal to the acceleration axis. A force transferring bar links the outer surface of the transistor case diaphragm with a pliable surface of a nickel alloy protective housing of each transistor, the bar being aligned along the acceleration or force axis. The pliable housing surfaces are held together by a support ring and maintain the physical arrangement of the transistors back-to-back. A change in velocity along the force axis causes an opposing reaction by the pliable surfaces which convey changes through the force transferring bar to flex the piezotransistor diaphragms, stressing the emitter-base junction of each transistor. In related electrical circuitry, the piezotransistors have collector outputs coupled to noninverting and inverting terminals of a differential amplifier. The emitters are coupled through a common current source which allows constant conduction through the piezotransistors. Acceleration forces coupled to the transistors produces a change in output voltage proportional to the specific force input over the linear range of operation. The predominant error-producing quadratic term and the bias component of the signal are canceled and the linear output is doubled without additional amplification.

United States Patent Little et al.. 5] Mar. 7, 1972 [54] COMPENSATEDOUTPUT SOLID- [57] ABSTRACT STATE DIFFERENTIAL A solid statedifferential accelerometer employing ACCELEROMETER piezotransistors formeasuring acceleration or velocity of a [72] Inventors: m m Joe 5. Hum",both of body along a specified axis of the accelerometer. A pair ofHuntsville, Ala. substantially identical piezotransisto'rs are arrangedback-toback along the axis through which tension and compressionAsslgneel The UIIIM sums 'lcl I8 forces of acceleration are applied. Apiezoelectrical emitterrepresented by the Secretary of the Army basejunction of each transistor is mechanically linked to a diaphragmportion of the transistor case that is normal to the [22] filed 1970acceleration axis. A force transferring bar links the outer sur- [2]App]. No.: 1,481 face of the transistor case diaphragm with a pliablesurface of a nickel alloy protective housing of each transistor, the bar1 being aligned along the acceleration or force axis. The pliable [52]73/517 R f housing surfaces are held together by a support ring andmain- Int. CI- min the arrangement of the transistors back.to.back,

0t 517, M A change in velocity along the force causes an opposingreaction by the pliable surfaces which convey changes through ReferencesCited the force transferring bar to flex the piezotrans'stor UNITEDSTATES PATENTS stressing the emitter-base junction of each 3,392,5767/1968 Hollander, Jr. ..73/5l7 X In related electrical circuitry, thePinon-amigo have collec- 3,386,292 6/1968 Watson et al. .73/517 A toroutputs coupled to noninverting and inverting terminals of 3,403,307 9/1968 Rmdnel' --317/235 M a difi'erential amplifier. The emitters arecoupled through a 3,512,054 5/ 1970 Owada etal ..317/235 M common gun-emsource which allows constant conduction Primary Examiner-James J. GillAttomey-Harry M. Saragovitz, Edward J. Kelly, Herbert Berl and Harold W.Hilton through the piezotransistors. Acceleration forces coupled to thetransistors produces a change in output voltage proportional to thespecific force input over the linear range of operation. The predominanterror-producing quadratic term and the bias component of the signal arecanceled and the linear output is doubled without additionalamplification.

5 Claims, 3 Drawing Figures PATENTEDMAR 71972 3,646,818

8 Lime a. Little Joe S.Hunfer,

INVEN oRs.

NONL|NEARITY BY FIG. 3 W aw COMPENSATED OUTPUT SOLID-STATE DIFFERENTIALACCELEROMETER BACKGROUND OF THE INVENTION In transducers employing thepiezoelectric effect to indicate acceleration, the direct effect ofmechanical stress applied to the transducer or sensor produces a changein the electrical charge on the faces of asymmetric crystals. Thus,tension or compression applied to the crystal generates a static voltageor voltage change across the crystal that is proportional to the stressor strain applied.

in obtaining accurate sensors and measurements therefrom, not only arehighly reliable and accurate electrical circuits required, but theoverall packaged instrument must be protected from varying environmentalconditions. Typical of the environmental conditions is broad temperaturedifferentials and rapid change from one temperature to the other, andconstant exposure to shock or vibration. Packaging of relativelyaccurate accelerometer sensors in small volumes results in a rather highunit cost. There is a considerable gap in technology in the area ofmedium accuracy characteristic and low-cost accelerometers and inpackage volume thereof.

SUMMARY OF THE INVENTION The apparatus of the present invention is adevice for measuring the rate of change of velocity and producing anoutput 7 voltage proportional thereto. The compensated output solidstate differential accelerometer can be fixed to a missile body or othermovable structure to provide velocity information in the direction offlight path, or in the direction wherein a monitor is required.

A pair of stress sensors are arranged in a back-to-back configurationand housed in a protective housing having a low temperature coefficientof expansion. The stress sensors are arranged coaxially with thedirection in which acceleration forces are to be measured. Axialacceleration forces cause tension and compression to be exerted onrespective sensors. An electrical response to these forces is coupledfrom each sensor to a differential signal detector wherein one of theresponse signals is inverted before the difference is obtained. Due tothe characteristics of positive and negative acceleration forces,tension and compression, the predominant errorproducing quadratic andbias component are canceled and the accurate linear output is doubled.This yields a stronger signal measurement that additionally overcomesinherent noise generations (such as vibrations) without amplification ofthe signal before application to integrators or other using circuitry.

An object of the present invention is to provide an accelerometer thatproduces an output proportional to the specific force input over thelinear region of operation while canceling error-producing signalcomponents.

Another object of the present invention is to provide an efficientaccelerometer having a sensing element packaged in a small volume.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a sectional view of thepiezoelectrical sensing element.

FIG. 2 is a schematic of the sensing element and related circuitry.

FIG. 3 is a graph illustrating the cancellation effect of errorproducingtenns by the sensor arrangement of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawingswherein like numerals represent like parts in all figures, there isdisclosed a preferred embodiment of the invention in FIGS. 1 and 2. FIG.1 discloses a sensing element having two piezotransistors or sensors l2and 14 arranged in a back-to-back configuration. Transistors l2 and 14are housed in separate and substantially identical support structures. Aquartz mounting ring 16 encompasses the circumference of transistor 12,being attached to the base structure of the transistor. A nickel alloyhousing 18, invar case, forms a partially cylindrical structure or casearound ring 16, being open at one end thereof and having the other endclosed by a pliable or flexible alloy housing portion. This flexibleportion of housing 18 serves as a diaphragm 20. A proofmass 22 providesa rigid link between the top 15 of transistor case 12 and the top ordiaphragm 20 of housing 18. Proofmass 22 is a mass that attempts to stayinertially fixed, resisting change, and may be a cylindrical rod fixedbetween structures 12 and 20, for example. Quartz ring 16 providescompensation for mismatch between accelerometer housing 18 and proofmass22 thermal expansion coefficients due to unequal lengths. Quartz ring 16has a much lower coefficient of thermal expansion than the nickel alloyof housing 18. The combined expansion of these elements, housing 18 andring !6, is set equal to the expansion of proofmass 22, therebypreventing undesired stresses being applied to piezotransistors I2 or14. Ring 16 also supports and protects the transistor.

Transistor 14 is supported and housed by similar structures including aquartz ring 16, housing 18, diaphragm 20 of housing 18 and proofmass 22.The diaphragms 20 of the respective housings are placed in mutualcontact with proofmasses 22 arranged substantially coaxial. An interfacering 25, having substantially the same coefficient of expansion ashousing 18, circumferentially encompasses the upper or top portions ofhousings 18 to hold them in a fixed position with respect to each other.The nickel alloy, invar, is used in the construction of housing 18,interface ring 25 and proofmass 22. This alloy is principally noted forits very low coefficient of thermal expansion (less than i microinch perinch per degree Fahrenheit).

The top portion 15 of piezotransistors I2 and 14 form one side portionof respective diaphragms within each transistor. The top portion ordiaphragm 15 is coupled to the transistor emitter-base junction (notshown). Stress signals that flex the top of either transistor arecoupled to the emitter-base junction thereof, stimulating apiezoelectric direct effect. When accelerometer 10 has an accelerationforce exerted thereon, transistor diaphragms l5, proofmasses 22 andhousing diaphragm 20 attempt to move against the acceleration thrustvector, along axis 28. Thus, placing one transistor diaphragm 15 intension and the other diaphragm 15 in equal but opposite compression.Force can be applied in either direction along axis 28 and overallsystem performance is unaffected, due to reversible response capabilityof the piezotransistor.

The electrical circuitry of the solid state differential accelerometer,shown in FIG. 2, includes piezotransistors l2 and 14 and a thirdtransistor 32. The emitters of transistors I2 and 14 are connectedtogether through a variable resistance 34. Collector load resistors 36and base biasing resistors 38 connect the base and collector oftransistors 12 and 14 to a common positive voltage source B+. Thecollector of transistor 12 is connected as the noninverting input 42 ofa differential amplifier 40 and the collector of transistor 14 isconnected as the inverting input 44 of amplifier 40. A resistor 46provides feedback from the output of amplifier 40 to the inverting lead44 and input resistors 48 establish the gain ratio of the amplifier. Thebases of transistors 12 and 14 are further connected through respectiveresistors 39 to system or circuit ground 50. A zener diode 52 andvoltage dropping resistor 54 combine with transistor 32 to provide aconstant current source for transistors 12 and 14. The anode of diode 52is connected to ground and to the emitter of transistor 32. The cathodeof diode 52 is connected to the base of transistor 32 and throughresistor 54 to 8+. The collector of transistor 32 is connected to thevariable arm 34a of variable resistor 34. The circuit ground 50 isfurther connected to amplifier 40. Amplifier 40 has an output terminal60 that connects an output signal to using circuitry such as integratorsfor measuring velocity.

In operation, accelerometer 10 may be aligned along the flight axis of amissile. The sensitive axis 28 or longitudinal specific force alongsensitive axis 28 results in a tension being exerted on one transistordiaphragm l5 and compression being exerted on the other diaphragm 15. Alarge reversible change is produced in the output characteristics when aforce is applied to the emitter-base junction of each transistor. Anunamplified linear output of approximately 20 percent of the supplyvoltage is achieved at the nominal maximum force input. The outputsignal is proportional to the specific force input over the linearregion of operation.

Diode 52 and resistor 54 maintain the base of transistor 32 at a fixedpotential andtransistor 32 is held in a conductive state. Transistors 12and 14 are conductively biased, conducting through trim resistance 34and transistor 32. Any unbalance in the system is indicated as an outputsignal on terminal 60 of amplifier 40. Before operation, this outputsignal is nulled to zero by adjusting variable resistance 34 so that thecollector signal on transistors 12 and 14 is identical. When a force isexerted along the sensitive or longitudinal axis of accelerometer theoutput voltages of transistors 12 and 14 change with respect to thetension or compression thereon. The differential signals on thecollectors of transistors 12 and 14 are coupled through resistors 48 toinputs 42 and 44 respectively of amplifier 40 wherein algebraic summingis provided. By inverting one input of amplifier 40 the bias andquadratic nonlinearity coefficients are cancelled and the useful linearoutput is doubled. Amplifier 40 provides a low impedance output atterminal 60 for connection to using circuitry.

With reference to FIG. 3 and assuming acceleration of accelerometer 10along the sensitive axis 28. Assuming transistor 12 to be in the foreposition, transistor 12 is in tension and transistor 14 is incompression. The horizontal axis of FIG. 3 represents the degree ofacceleration in the positive or negative direction and the vertical axisrepresents the amount of nonlinearity of the transistor output signalwith increasing degrees of acceleration; Curve A is the cubic andquadratic composite output of piezotransistor 14, which is subjected tonegative acceleration. Curve B is the inverted representation of curve Aperformed by the inverting input of amplifier 40. Curve C is the cubicand quadratic composite output of piezotransistor 12, exposed topositive acceleration. Curves A and C are representative of the outputsof stress sensitive transistors in response to stresses applied to theemitter-base junction thereof. Curve D is the algebraic summation ofcurves B and C. ln mathematical notation, the output (A) correspondingto curve A is written as K Accelerometer bias tenn K, Scale factor K,Quadratic nonlinearity coefficient K Cubic nonlinearity coefficient Kn'" order nonlinearity coefficient, and

a Applied acceleration.

Since terms beyond the quadratic coefficient contribute very little tooverall nonlinearity, the equation representing curve A is truncatedwith the K coefiicient and is written as K,, K a K,a =A.

inverting this equation results in the equation for curve B,

K., K a K a-=B. 2 Similarly, the truncated equation for curve C isrepresented y.

K K,a +K a"=C. (a Summing the last two equations algebraically resultsin an output equal to,

Thus, both the bias and the quadratic nonlinearity coefficient arecancelled and the useful linear output is doubled.

A working model of this circuit can be constructed with the nickel alloycomponents formed of UNlSPAN LR35 and having the following typicalcomponent values:

Trunliltors l2, l4 PITRAN Model PT-3 Trnnnlstor 32 Motorola 2N2222Amplifier 40 Fairchild 709 Resiltors 38 68K ohms Resistors 36 75K ohmsResistors 39 3.3K ohms Resistors 48 lOK ohms Resistor 46 lOOK ohmsResistor 34 I00 ohm Potentiomcter with 5K ohm bridge.

The compensated output solid-state differential accelerometer producesan output voltage which is proportional to rate of change of velocity.The accelerometer can be fixed to a missile body or other body toprovide velocity information therefor in a specific direction. If it isdesired to measure accelerations along a lateral plane, theaccelerometer can be mounted on a stabilized platform. By stabilizingsuch a platform with three gyros, effective along the yaw, roll andpitch axes, its direction in space can be controlled. With this typicalarrangement, it is possible to utilize the accelerometer alonglongitudinal and lateral axes to determine either velocity by a singleintegration or distance travelled by two successive integrations. Theaccelerometer can be used to provide data for cutting off a rocket motorat the desired point in a flight trajectory. lt can be mechanized tomake very accurate gravity measurements, and can also be used as an axesleveling device in north-seeking platform applications.

We claim:

1. A solid-state differential accelerometer for measuring accelerationof a body along the sensitive or longitudinal axis of saidaccelerometer, comprising: first and second piezotransistors havingrespective output electrodes and arranged in a back to-backconfiguration along said sensitive axis, said piezotransistors beinghoused in respective protective containers, said containers each havinga diaphragm wall portion thereof; first and second housing means havinga pliable surface area and partially encompassing said first and secondpiezotransistor containers respectfully; first and second forcetransmitting proofmasses respectively connecting the diaphragm wallportion of said first and second piezoelectric containers to theinterior of said pliable surface of said first and second housing meansfor applying pressure to said piezotransistors in response toacceleration forces, said pliable surface and container diaphragm wallportion being arranged normal to said sensitive axis, said proofmassesbeing coaxially aligned along said sensitive axis; and a differentialamplifier having an inverting and a noninverting input connectedrespectively to said output electrodes of said first and secondpiezotransistors, and an output for connection to indicating or usingcircuitry.

2. An accelerometer as set forth in claim 1 wherein each of said housingmeans is a nickel alloy, and further comprising an additional nickelalloy ring around the circumference of said first and second housingalloys for maintaining said housings pliable surfaces in mutual contactand thereby allowing acceleration forces along said longitudinal axis toexert tension on one electron device container and compression on theother simultaneously.

3. An accelerometer as set forth in claim 2 and further comprising aquartz mounting ring around the circumference of each of saidpiezotransistor containers for protection and support thereof, saidcontainers being adjacent the inner surface of said rings and saidhousing being fixed to the outer surface of said rings.

4. An accelerometer as set forth in claim 3 wherein saidpizeotransistors are NPN-planar transistors and further comprising avariable resistance connected between the emitters of said transistors,a constant current source connected through said variable resistance tosaid emitters, a power source connected respectively through load andbias resistors to the collector and base of said transistors, and saidoutput electrodes being the collector of each of said transistors.

5. An accelerometer as set forth in claim 4 wherein said constantcurrent source is a third NPN-transistor having the collector connectedto said variable resistance, the base connected through a resistance tosaid power source, and a zener diode connected in reverse between thebase and emitter thereof and said emitter being further grounded to acircuit common.

1. A solid-state differential accelerometer for measuring accelerationof a body along the sensitive or longitudinal axis of saidaccelerometer, comprising: first and second piezotransistors havingrespective output electrodes and arranged in a back-to-backconfiguration along said sensitive axis, said piezotransistors beinghoused in respective protective containers, said containers each havinga diaphragm wall portion thereof; first and second housing means havinga pliable surface area and partially encompassing said first and secondpiezotransistor containers respectfully; first and second forcetransmitting proofmasses respectively connecting the diaphragm wallportion of said first and second piezoelectric containers to theinterior of said pliable surface of said first and second housing meansfor applying pressure to said piezotransistors in response toacceleration forces, said pliable surface and container diaphragm wallportion being arranged normal to said sensitive axis, said proofmassesbeing coaxially aligned along said sensitive axis; and a differentialamplifier having an inverting and a noninverting input connectedrespectively to said output electrodes of said first and secondpiezotransistors, and an output for connection to indicating or usingcircuitry.
 2. An accelerometer as set forth in claim 1 wherein each ofsaid housing means is a nickel alloy, and further comprising anadditional nickel alloy ring around the circumference of said first andsecond housing alloys for maintaining said housings pliable surfaces inmutual contact and thereby allowing acceleration forces along saidlongitudinal axis to exert tension on one electron device container andcompression on the other simultaneously.
 3. An accelerometer as setforth in claim 2 and further comprising a quartz mounting ring aroundthe circumference of each of said piezotransistor containers forprotection and support thereof, said containers being adjacent the innersurface of said rings and said housing being fixed to the outer surfaceof said rings.
 4. An accelerometer as set forth in claim 3 wherein saidpizeotransistors are NPN-planar transistors and further comprising avariable resistance connected between the emitters of said transistors,a constant current source connected through said variable resistance tosaid emitters, a power source connected respectively through load andbias resistors to the collector and base of said transistors, and saidoutput electrodes being the collector of each of said transistors.
 5. Anaccelerometer as set forth in claim 4 wherein said constant currentsource is a third NPN-transistor having the collector connected to saidvariable resistance, the base connected through a resistance to saidpower source, and a zener diode connected in reverse between the baseand emitter thereof and said emitter being further grounded to a circuitcommon.